1. Field of the Invention
This invention relates to an apparatus and method for delivering fluid with a controlled rate of flow. More particularly this invention relates to a simple, compact, inexpensive infuser which is capable of delivering a relatively constant dosage of an active agent at a controlled rate for parenteral delivery, and to a method for achieving such delivery of fluid.
2. Description of the Prior Art
Active agents are commonly administered to a subject parenterally either by injection or infusion. The commonly used distinction between the two is that an injection introduces a dosage of drug in a pulse form, either subcutaneously, intramuscularly, intravenously, or intraarterially, whereas an infusion introduces the active agent in a continuous manner over a prolonged period of time, such as, for example, in the conventional intravenous drip. It is generally more desirable to eliminate pulse dosages in which a relatively large amount of the active agent is rapidly deposited for consumption in the body over a prolonged period of time, in favor of the administration over long periods of time of smaller doses. Further it is highly desirable to infuse active agents to the patient at a constant, controlled rate.
The usual procedure for delivering fluids with a controlled rate of flow, for example, in the gradual parenteral administration of biological fluids, such as blood, plasma or medicinal solutions, such as drugs into the human body, was to use gravity-operated apparatus such as an intravenous infusion set. An intravenous infusion set comprises a bottle for the fluid which is supported in an inverted position at a higher level than the patient, an intravenous feed tube, typically of a plastic material and a resistor mechanism, such as a clamp operating on the plastic tube, the clamp being adjusted so as to allow the fluid to drip at a controlled rate into a drip chamber located below the bottle. The drip chamber allows a nurse or other attendant to observe the rate of flow of fluid from the bottle and to adjust the resistor to give a desired rate. It also creates a reservoir for the fluid at the lower end of the chamber to ensure that no air enters the main feeding tube leading to the patient. Because it is necessary to maintain the drip bottle at a substantially constant height above the patient, the conventional intravenous set is inconvenient for mobile use.
While an intravenous infusion set has several virtues, including simplicity, independence from external power supplies and adjustability over a wide range of flow rates, it is not entirely satisfactory because it is subject to variations in the flow rate produced. It is therefore necessary that the flow rate of the fluids delivered by the intravenous set be monitored frequently and adjusted as necessary to maintain the desired rate.
Various equipment has been designed to provide better control of flow rate, including, for example, devices which use servo controlled systems to drive a power unit for a pump, and to power in-line linear actuators which control the opening on the intravenous set. Such equipment is both complex and expensive.
More recently, self-driven pumps have been developed for the administration of active agents by infusion. One example of such a device is a pump driven by a constant force spring powered drive to provide a constant rate of fluid discharge, described in U.S. Pat. No. 4,202,333. This device is relatively expensive and the device is cumbersome to transport and is therefore cumbersome when used by ambulatory patients. Another such device incorporates a special constant rate elastomeric balloon in combination with a special intravenous set which includes a rate controlling orifice, described in U.S. Pat. Nos. 4,318,400 and 4,386,929. These devices are not entirely satisfactory because of the significant manufacturing tolerances involved to meet product functional requirements. For example, the physical properties and dimensions of the elastomeric balloon must be carefully controlled by provide the desired controlled flow rate. Changes in rubber durometer or tube wall thickness can materially affect fluid delivery rate. Further, the delivery rate may be adversely affected by variations in pressure on the elastomeric balloon resulting from changes in external pressure such as atmospheric pressure. Also, the elastomeric balloon may burst in actual use if not properly formulated, and this can be extremely hazardous, particularly in the case of chemotherapeutic drugs and the like. Additionally, the elastomeric balloon is not readily adapted to factory pre-filling and extended storage, particularly of medicinal fluids, because of chemical interaction between the elastomeric balloon and the fluid.
Another example of an apparatus for delivering fluids with a controlled rate of flow is described in U.S. Pat. No. 4,180,067. The apparatus includes an "actuator" which includes a vacuum power unit. The vacuum power unit includes a cylinder which is closed at one end and has a piston slidable therein. The piston in the cylinder of the vacuum power unit is coupled to a movable wall of a liquid storage container through a cross-head 15 which functions as a transmission mechanically coupling the separate piston of the liquid storage container to the piston in the cylinder of the vacuum power unit. The cylinder of the vacuum power unit is evacuated when the slidable piston is substantially at the open end of the cylinder so that atmospheric pressure acts upon the piston to drive it towards the closed end of the cylinder. That motion produces a reduction of the volume of the liquid storage container through the cross-head transmission which couples the piston in the vacuum cylinder to the movable wall of the container. This apparatus is rather cumbersome and complicated. The vacuum power unit and liquid storage unit are configured parallel to one another, and the apparatus relies on a two piston arrangement in combination with the transmission means connecting the two separate pistons to effect delivery of the liquid from the storage container. The apparatus does not provide a single plunger means to provide the power drive means and to effect discharge of the liquid from the apparatus.
Thus there remains a need for a simple, compact, inexpensive infuser which is capable of delivering fluid at a relatively constant and controlled rate of flow, which may be prefilled with medicinal liquids and stored, and which may be used by ambulatory patients.
Accordingly, it is a principal object of the present invention to provide an apparatus for delivering fluid at a relatively constant, controlled rate of flow. It is a related object to provide such an apparatus for delivering a medicinal liquid.
It is a further object of this invention to provide such an apparatus that is useful for introducing small amounts of a medicinal liquid to a patient over a prolonged period of time.
It is yet another object of this invention to provide such an apparatus that is compact and simple. A related object is to provide such a device that is portable and not cumbersome and is thus suitable for use by ambulatory patients.
It is a further object of the present invention to provide an apparatus for delivering fluid at a relatively constant, controlled rate of flow which can be prefilled with medicinal fluids and stored for an extended period of time, ready for use.
These and other objects of the invention will be readily apparent from the following description with reference to the accompanying drawings.